I'm a big fan of the genre, so I have a lot to choose from. A question: does this only cover games from the "classic" era, or are modern text adventures made in classic formats okay?
If the first: the original Adventure (aka Colossal Cave) by Crowther and Wood, definitely. Yes, it can be overly unfair and hard, but it was the first! The atmosphere and writing are still darn good, and it has some fine puzzles.
If modern text adventures (late 90s and beyond) are included: like I said, a difficult choice, but one of my top picks would be Metamorphoses by Emily Short. It's short-ish, but very well coded, with a system for transforming or re-sizing objects that enables a great scope for different puzzle solutions. Plus, it may have the finest writing in any text adventure I've played.